1. Field of the Invention
The present invention relates to a light beam scanning apparatus used for a copying machine, laser printer, or the like.
2. Description of the Related Art
In a laser printer or the like using an electrophotographic method, a charger charges a photosensitive member as an image carrier, and a latent image is formed on the photosensitive member by laser application upon driving a semiconductor laser in accordance with image information. An image is then formed on a sheet or the like by transferring, onto the sheet, a toner image obtained by developing the latent image using the developing agent.
In such a laser printer, a collimator lens converts a laser beam emitted by a semiconductor laser used as a light source into almost parallel light. A deflecting member such as a rotary polyhedral mirror (polygon mirror) deflects the parallel light at a predetermined beam diameter. An f-θ lens then focuses the resultant light. At the same time, the f-θ lens corrects the distortion so as to guarantee the temporal scanning linearity. The laser beam having passed through the f-θ lens is focused and scanned on the photosensitive member in the main scanning direction (axial direction of the photosensitive member) at a uniform velocity. At this time, the surface of the photosensitive member is scanned with a desired spot. A shift in focusing position on the photosensitive member causes degradation of the image quality. An implementation is made to prevent the focusing position (beam waist) of the laser beam from greatly shifting on the photosensitive member. For example, the mechanical precision and arrangement precision of optical components and mechanical components are improved, and the coefficients of linear expansion of these components are optimized.
Conventionally, however, image plane curvature occurs, in which the position of a beam waist meanders in the direction of the optical axis depending on the scanning position of a laser beam on the photosensitive member, or the position of a beam waist varies owing to variations in the relative distance between components due to changes in temperature or owing to changes in refractive index.
Recently, as color laser printers have become popular, there have been increasing demands for the density uniformity of highlighted halftone images.
In a laser printer, in order to properly reproduce a highlighted halftone image, it is necessary to form a small spot with a uniform spot profile on the photosensitive member surface. In addition, it is preferable that the profile of the spot not vary with time.
With regard to such a demand, Japanese Patent Laid-Open No. 58-57108 discloses a technique of improving the image formation characteristic of an optical system by allowing an actuator to move the position of a lens so as to correct the beam waist position of a scanning beam.
According to Japanese Patent Laid-Open No. 4-264420, as shown in FIGS. 18A and 18B, a cylindrical lens 103 intermediately forms a laser beam 111 into an image before it is deflected by a polygon mirror 107, and an electro-optical element 104 is placed at the beam waist portion. According to this proposal, the image formation characteristic of the optical system is improved by applying a voltage to the electro-optical element 104.
As shown in FIG. 19, the electro-optical element 104 has a pair of electrodes 113 made of Au or the like which are arranged on opposing surfaces of an electro-optical crystal 112, and changes the refractive index of the electro-optical crystal 112 by changing the electric field strength by applying a voltage in a direction perpendicular to an optical axis 52 by using a control power supply 110.
The technique disclosed in Japanese Patent Laid-Open No. 58-57108 is inferior in the stability of the positions of optical parts and reproducibility to the technique of fixing optical parts, and is difficult to move the optical parts at high speed. This makes it difficult to correct image plane curvature.
According to the technique disclosed in Japanese Patent Laid-Open No. 4-264420, the refractive index profile produced at the electro-optical element 104 is not constant. For this reason, the uniformity of a latent image is impaired by variations in the light amount profile of a spot on the scanned surface, or a light beam is deflected in accordance with the magnitude of an electric field applied to the electro-optical element 104.
In addition, since an intermediate beam waist portion is formed by the electro-optical crystal 112, a complicated lens arrangement is required.